The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used for the storage and retrieval of digital data, and for the sensing of external magnetic fields.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as magnetic field sensors. Digital data memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital symbols as alternative states of magnetization in magnetic materials provided in each memory storage cell, the result being memories which use less electrical power and do not lose information upon removals of such electrical power.
Such memory cells, and magnetic field sensors also, can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
Ferromagnetic thin-film memory cells, for instance, can be made very small and packed very closely together to achieve a significant density of information storage, particularly when so provided on the surface of a monolithic integrated circuit. In this situation, the magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin-film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides significant “flux closure,” i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in the cell to affecting primarily just that cell. This result is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin. Similar “sandwich” structures are also used in magnetic sensors.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended “sandwich” structures, and adding possibly alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a “giant magnetoresistive effect” being present in some circumstances. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude or more greater than that due to the well known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response, varying the difference occurring between the direction of the magnetization vector in a ferromagnetic thin-film and the direction of sensing currents passed through that film leads to varying effective electrical resistance in the film in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the field and the current direction therein are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance in such a magnetoresistive ferromagnetic film can be shown to be given by a constant value, representing the minimum resistance, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance has a magnitude characteristic that follows the square of the cosine of that angle.
Operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film. Such an axis comes about in the film because of an anisotropy therein typically resulting from depositing the film during fabrication in the presence of an external magnetic field oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of the device having this resulting film, such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film's easy axis. The state of the magnetization vector in such a film can be measured, or sensed, by the change in resistance encountered by current directed through this film portion. This arrangement has provided the basis for a ferromagnetic, magnetoresistive anisotropic thin-film to serve as a memory cell.
In contrast to this arrangement, the resistance in the plane of a ferromagnetic thin-film is isotropic for the giant magnetoresistive effect rather than depending on the direction of the sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect involves a change in the electrical resistance of the structure thought to come about from the passage of conduction electrons between the ferromagnetic layers in the “sandwich” structure, or superlattice structure, through the separating nonmagnetic layers with the resulting scattering occurring at the layer interfaces, and in the ferromagnetic layers, being dependent on the electron spins. The magnetization dependant component of the resistance in connection with this effect varies as the sine of the absolute value of half the angle between the magnetization vectors in the ferromagnetic thin-films provided on either side of an intermediate nonmagnetic layer. The electrical resistance in the giant magnetoresistance effect through the “sandwich” or superlattice structure is lower if the magnetizations in the separated ferromagnetic thin-films are parallel and oriented in the same direction than it is if these magnetizations are antiparallel, i.e. oriented in opposing or partially opposing directions. Further, the anisotropic magnetoresistive effect in very thin films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas a significant giant magnetoresistive effect is obtained only in very thin films. Nevertheless, the anisotropic magnetoresistive effect remains present in the films used in giant magnetoresistive effect structures.
A memory cell based on the “giant magnetoresistive effect”, or GMR effect, can be provided by having one of the ferromagnetic layers in the “sandwich” construction being prevented from switching the magnetization direction therein from pointing along the easy axis therein in one to the opposite direction in the presence of suitable externally applied magnetic fields while permitting the remaining ferromagnetic layer to be free to do so in the same externally applied fields. In one such arrangement, a “spin-valve” structure is formed by providing an antiferromagnetic layer on the ferromagnetic layer that is to be prevented from switching in the externally applied fields to “pin” its magnetization direction in a selected direction. In an alternative arrangement often termed a “pseudo-spin valve” structure, the ferromagnetic layer that is to be prevented from switching in the externally applied fields is made sufficiently thicker than the free ferromagnetic layer so that it does not switch in those external fields provided to switch the free layer.
Thus, a digital data memory cell based on the use of structures exhibiting the giant magnetoresistive effect is attractive as compared to structures based on use of an anisotropic magnetoresistive effect because of the larger signals obtainable in information retrieval operations with respect to such cells. Such larger magnitude signals are easier to detect without error in the presence of noise thereby leading to less critical requirements on the retrieval operation circuitry.
Typically, such GMR effect based magnetoresistive memory cells are provided as an array thereof in and from which data is stored and retrieved, the array being formed on an insulating layer provided on a substrate containing a monolithic integrated circuit for operating the cells. These memory cells are typically interconnected by sense line interconnections in parallel series strings so that each such string forms a sense line. In addition, cells these strings are typically provided separated by portions of electrically insulating layers from a first set of word lines that extend parallel to one another and perpendicular to the sense lines so that each of these word lines passes under a corresponding sequential set of cells such that each cell in the set is in a different sense line string, and further separated by portions of electrically insulating layers from a second set of word lines each provided parallel to a corresponding sense line usually on a side of the cells opposite that adjacent to the first set of word lines. Alternatively, the word line sets can be perpendicular to one another but on diagonal axes with respect to the sense lines.
These sense lines and word lines together allow coincident current pulses therein to generate coincident magnetic fields thereabout that combine at selected ones of the memory cells where a current activated sense line and current activated word lines cross one another. Data can be stored in such cells when coincident currents generating external magnetic fields are used in the word lines only although such fields can be enhanced by using coincident currents in the appropriate sense lines also. Data sensing currents for retrieving stored data from the cells are established in those cells through the sense lines.
One such digital data memory cell, 5, shown in a top view of a monolithic integrated circuit portion in FIG. 1 utilizes the vertical “giant magnetoresistive effect” in which sense currents through the cell follow a path perpendicular to the planes in which the cell thin-film layers are provided as opposed to the version of the effect described above with currents being parallel to those planes. A cross section view, as marked in FIG. 1, is shown as a resulting layer diagram of that integrated circuit portion in FIG. 2 but with the view there being tilted upward slightly. GMR based cell 5 is fabricated of sequentially deposited multiple thin-film layers to form a stack, 6, thereof that are patterned during such fabrication to each have the shape of a circular disk with a centered circular opening therein. The magnetic material layers in the stack are alternately provided relatively thick (providing relatively harder magnetization) and relatively thin (providing relatively softer magnetization), and the stack is formed of repeats of copper and then a soft magnetic material layer followed by copper and then a hard magnetic material layer. The magnetic material layers are a ferromagnetic alloy of nickel, iron and cobalt, with a composition of Ni(65%)Fe(15%)Co(20%).
Electrical contacts, 7A and 7B, are provided on the top and bottom, respectively, of stack 6 in each cell 5 such that any sense current flows through the cell are established perpendicular to the planes of the cell stack films. A number of such memory cells 5 can be interconnected beginning, for example, with top electrical contact 7A of one cell being interconnected to top electrical contact 7A of a subsequent cell with the interconnection involved also being designated as 7A, and then from bottom contact 7B of this subsequent cell to bottom contact 7B of the next subsequent cell similarly designating the interconnection also as 7B, the result being a series connected string of such cells to form a sense line, 7.
Word lines, 8A, 8B, 8C and 8D, are formed by two pairs of parallel conductors with one pair, having word lines 8A and 8B therein, positioned over the top of interconnected cell stack 6 and the other pair, having word lines 8C and 8D therein, positioned below the bottom of interconnected cell stack 6, respectively, and these pairs of word lines are arranged in those positions to extend orthogonally to each other by pair. Word line currents are typically established in each conductor within a pair having the same magnitude in each but flowing in opposite directions to one another. Such currents in the paired word lines generate magnetic fields encircling them to result in an approximation of radial magnetic fields being established through the corresponding memory cell in having the field due to each line, directed either outward or inward depending on current direction, in the half of the thin-film disks having a substantial fraction thereof being crossed by the corresponding word line. The current directions shown in FIG. 2 lead to magnetic fields represented there by closed dotted lines ellipses with arrow heads shown along those dotted lines to indicate field direction. An estimate of the field strength at stack 6 due to such a current I in these word lines for the word lines having a width of W and a separation from that stack of d on a center-to-center basis is given by Itan−1[(W/2d)/(0.08πW)]. Any sense line currents flow from top to bottom, or vice verse, through memory cells 5 as indicated above thereby generating corresponding circular magnetic fields in the planes of the film layers provided in the cell stacks.
The toroidal shaped memory cells resulting from such stacked thin-films therein forces magnetizations in the magnetic materials of those films to follow circular paths thereby enabling stable flux path closure within the cells. The combination of radial magnetic fields generated by word line currents and the circular magnetic fields generated by sense line currents produces very robust and repeatable switching of the magnetization in stack 6 of memory cell 5 in opposite circular directions following the cell toroidal walls.
The hard magnetic material layers in thin-film layer stack 6 of cell 5 can be switched in magnetization direction together for a relatively large externally applied magnitude magnetic field (along with the other magnetic layers, that is, the soft layers) and the soft magnetic material layers can be switched together for a relatively small externally applied magnetic field (without also switching the hard layers). A memory state is stored in stack 6 of cell 5 as the magnetic materials magnetization directed in one of two opposite circular directions around the center opening in the hard magnetic material layers therein. The soft magnetic material layers in the stack serve for providing the capability to perform an “interrogating” function in the detecting of which of the circular directions in the cell walls the magnetization of the hard layers is currently oriented.
Thus, to retrieve a current memory state of stack 6 of cell 5, two sequential current pulses with opposite polarities can be applied to cell sense line 7 along which that cell is positioned with there also being applied a word line current through the word line adjacent to that cell with a magnitude chosen to be at the state retrieval value. The magnitude of the current pulses is such that only the one of cells 5 having an adjacent word line with current therethrough can have its soft magnetic material layers switched in magnetization circular direction. The occurrence of such a switching of the soft magnetic material layers in magnetization direction for one of these current pulses, with the hard magnetic material layers retaining their current magnetization direction, will change the resistance through the chosen cell 5 to currents in the corresponding sense line 7 thereby signaling the cell magnetic state or memory state.
As stated and described above, operating magnetic fields imposed externally through establishing electrical currents in nearby conductors, or even in the cells themselves, can be used to vary the angle of the magnetic materials magnetization vector with respect to the easy axis, or magnetically unperturbed axis, in the ferromagnetic films of these various kinds of memory cell devices, particularly the free layers. Such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the layer magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the easy axis of the layer, the state of the cell determining the value of the binary bit being stored therein. One of the difficulties in such memories is the need to provide memory cells therein that have relatively uniform switching thresholds and adequate resistance to unavoidable interjected magnetic field disturbances in the typical memory cell state selection scheme used. This externally applied operating fields scheme is based on selective externally imposed magnetic fields provided by selectively directing electrical currents near by or through sequences of such cells, or both, thereby giving rise to such magnetic fields so that selection of a cell occurs through coincident presences of such fields at that cell.
In such a coincident current selection arrangement, only that cell in the vicinity of the crossing location, or intersection, of these paths (one over a sequence of cells and another through another sequence of cells, and perhaps yet another below a further sequence of cells) experience sufficient magnetic field intensities due to the summing of the fields there, arising because of these coincident currents, to cause such a magnetic state change therein. Cells in the array that are located far away from these current paths are not significantly affected by the magnetic fields generated by such currents in the paths because such fields diminish in intensity with distance from the source thereof. Cells, however, located in relatively close proximity to one, but not two or three, of these paths do experience more significant magnetic fields thereabout, and those immediately in or adjacent to such a path experience sufficient field intensities to be considered as being “half-selected” in two path arrangements (or a “third-selected” in three path arrangements as described above for memory cells 5) by the presence of current in that path intended to participate in fully selecting a different cell along that path at the intersection with the other path or paths on which a selection current is also present. Half-selection or third-selection means that a bit is affected by magnetic fields from the current through the one path but not by those coincident currents present on another path or paths. Such a coincident interjected magnetic fields memory cell state selection scheme is very desirable in that an individual switch, such as that provided by a transistor, is not needed for every memory cell, but the limitations this selection mode imposes on the uniformity of switching thresholds for each memory cell in a memory make the production of high yields difficult.
As such magnetic thin-film memory cells are made smaller to thereby increase the cell density over the surface of the substrate on which they are disposed, the resulting cells become more subject to magnetic state, or data, upsets due to thermal fluctuations occurring in the materials therein. The depth of the energy well in the magnetic material of such cells can be approximated as Hweff*Ms*Volume, where Hweff is half the effective restoration magnetic field attempting to maintain the current magnetic state following perturbations thereto and so effectively providing the energy well depth, Ms is the saturation magnetization of the magnetic material in the cell, and Volume is the volume of the magnetic material in the cell. In conventional cells, Hweff is provided by shape anisotropy or anisotropy due to the material properties of the cell magnetic material, or both. Typically, the value of Hweff in these cells is less than 100 Oe.
Toroidal stacks 6 in memory cells 5 described above, in having orthogonal word line sets positioned on opposite sides thereof, have a further cell magnetic state upset possibility if electrical current is established in only one of these sets of word lines in a third-select situation. In such occurrences as part of fully selecting some other cell, the third-select cells do not have a full radial magnetic field established thereabout. The field that is established thereabout has much less symmetry and this can result in forming unwanted domain walls in the magnetic materials in such third-select cells during such occurrences which leads to “magnetic noise” and the possibility of erroneous state information being obtained form the cell.
In addition to risking such upsets, the use of two orthogonally positioned sets of word lines in addition to sense lines requires that a switch or multiplexer be used with a second word line set current supply in conjunction with the current supply for operating the first word line set, and perhaps in conjunction with the sense lines, as a cell selection is made through supplying currents to the orthogonally positioned word lines adjacent thereto and possibly also the corresponding sense line. Thus, there is a desire for a cell selection arrangement with reduced upset risks, reduced operating circuitry and reduced operating currents which otherwise cause additional unwanted device heating.